This invention relates generally to high-speed networks that carry digital video traffic and to video switching equipment that may be used in the television/video broadcast industry. The concepts presented herein are best understood with reference to a simple television broadcast example, pictured sequentially in FIGS. 1A, 1B, 1C and 1D.
Pictured in FIG. 1A are certain elements of a television news analog broadcast system as they relate to the switching of live video feeds. In that broadcast system are a number of video feeds; here there are four numbered 10a through 10d. Each of these feeds is input into a video switch 11, which for simplicity of the example is pictured as a matrix crossbar. Switch 11 is controlled from user controls 12, which in this example is a set of four switches. There are two video outputs of switch 11, which include a transmit feed 13a connected to a transmitter 14 and a monitor feed 13b, which may be distributed to those controlling the system.
In this exemplary newscast configuration, feed 10a is from a camera directed to an anchorwoman, feed 10b is from a camera directed toward an anchorman, feed 10c is directed to a first playback device having loaded thereon a first video clip, and feed 10d is likewise directed to a second playback device having a second video clip. In the state shown in FIG. 1A control 12 is configured to selection “1”, wherein the output feed 13a is set to the anchorwoman feed 10a and the monitor feed 13b is set to the first playback device feed 10c. This configuration could be used where the anchorwoman was introducing a video clip loaded onto the first playback device.
Momentarily following the anchorwoman's introduction, a person controlling the system presses “2” on the control 12, causing switch 11 to reconfigure into the state shown in FIG. 1B. There, transmit feed 13a is connected to the first playback feed 10c, thereby playing the first clip. In such a system, the pressing of “2” might also cause the first playback device to begin playing the first clip synchronously. Monitor feed 13b is now receiving the anchorman feed 10b in preparation for a switch to him following the first clip.
When the first clip completes either a person controlling the system or automation selects the third state, shown in FIG. 1C. There, the transmitting feed 13a is receiving the anchorman feed 10b, transmitting his introduction, and monitor feed 13b is connected to the second playback feed 10d. While this is happening, a new video clip is loaded into the first video playback device, which is shown on its corresponding feed 10c. 
When the anchorman is finished, the controlling person selects “4” on the controlling device to bring the system into the state shown in FIG. 1D, with the transmitting of the second video feed 10d and the monitor 13b set back to the anchorwoman feed 10a in preparation for the next transition back to the state shown in FIG. 1A.
Now although this example is far too simple for real newscast practice, it serves to illustrate several important concepts. First, although the exemplary newscast system could be automated, it is generally undesirable to do so. Newscasts are planned events, with a number of news stories selected and scheduled for time. However, unless portions of the newscast are pre-recorded, it is generally not known how long segments will be. This is particularly true of newscasts, where anxiety, a cough or any other number of factors can cause a person under live transmission to shorten or lengthen the desired length of their segments. Thus, if the length of a live segment were set in advance, a person being broadcast might have his words cut off at the end or might end too quickly leaving an uncomfortable pause.
Note however, in the example presented here, that some portions of the broadcast can be automated, namely the transitions following a pre-recorded clip. In fact, for prepared materials automation may be more desirable to avoid human-introduced errors into the broadcast.
The last point made in connection with this example is that switches, such as 11, vary widely and are normally configurable by the end-user. Thus, the connections of the crossbar of switch 11 would have been configured in advance of the news broadcast for the sequence illustrated in FIGS. 1A through 1D, which for modem equipment would be performed by programming rather than a mechanical setup.
Now referring to FIG. 2, although video switching systems vary widely, they all have the following elements. First, there are a number of video feeds 20a to 20n, which number will vary depending on the purpose for which the system is used. For example, a video production company might have a largely self-contained system having less than ten total video feeds. On the other hand, a satellite or cable broadcasting company would likely have hundreds or even thousands of feeds. It will be desired to select from those feeds and consume a portion of those feeds in consumers 22a to 22n. Those consumers might be, for example, transmitters, modulators, monitors or recording devices. One or more video switches 21 will be configured to select the video feeds in accordance with one or more controlling devices 23. Here although in a simple setup only one video switch 21 and one control device 23 are shown, it is to be understood that any of the video switching systems described herein might incorporate any number of those devices, for example by daisy-chaining or other techniques.
The examples given so far are especially suitable for analog video transmission, in for example the NTSC or PAL standards. In that mode of transmission, simple switches and relays could be used to implement the switching mechanisms. Today, such switches would use a synchronization signal to avoid switching between video signals in different vertical phases, which signal would be distributed throughout a broadcaster's equipment.
In recent years, digital video transmission has become widespread. Note first, that a system as shown in FIG. 2 could be used for switching between digital broadcast feeds, if desired. Note that, regardless of whether analog or digital video is used, the number of wires or cables 24 is equal to the number of consumers 22a through 22n connected to the switch 1. Thus, a television broadcaster's building would be required to have installed therein many runs of cable to support the wide variety of configurations desired.
The development of high-speed packet-switched networks and digital video has brought certain new developments to video broadcasting. By using a digital network as the conduit for digital video, the duplicity of cabling required by the systems as for FIG. 2 can be avoided. Furthermore, wired and fiber-optic versions of ethernet are now available at a speed of one billion bits per second. With that high bandwidth available, is now possible to transmit several compressed digital video streams effectively over a single wire, which in practice can reduce the number of physical cables of video broadcaster must install by many times.
The system shown in FIG. 2 can be modified to utilize a packet switched network, which modifications are shown in FIG. 3. There, a control device 33 controls a video switch 31 that receives a number of incoming digital video feeds 30a to 30n. Video switch 31 buffers each of feeds 30a to 30n generally so that a frame is available for each feed in the event a switch is made. Alternatively, video switch 31 could receive analog video from one or more of video inputs 30a to 30n and digitally encode those signals. Video switch 31 contains a processor that packetizes the selected video streams and sends that data to a network backbone 35, optionally by way of a network switch or router 36. Further in FIG. 3, two sets of video consumers 32a to 32n and 32s to 32z are connected to network backbone by way of network switch routers 34a and 34b. 
FIG. 3 shows a best case scenario for a backbone-based digital video distribution network. This network is efficiently used because there is in effect only one device depositing network traffic, namely video switch 31. As shown, there are no other connections to the video distribution network, which can be done by providing the network infrastructure isolated from other digital networks that may exist in a building. Were additional video sources to be added to backbone 35 network collisions would occur, which would reduce throughput in the network to as little as one third capacity. Assuming that a compressed digital video stream of 20 Mb per second was standard, the capacity of a single gigabit backbone would support a maximum of 16 concurrent video streams. Furthermore, in that system there would be no assurance of fairness of delivery, and packet latencies could vary so much that video data would not necessarily be timely delivered to a consumer. This may be sufficient for a small size video production house that needs only a medium number of concurrent video streams, but is not sufficient for a large broadcaster that may have 20 or more concurrent streams operating at the same time.
To date, the solution to this problem has been to segment a video distribution network into several areas and/or backbones. However, it is desirable to accommodate as many streams on a single network as possible. For example, many buildings are wired to operate a single 10BaseT network, and operating a second network may require a restringing of the wires, cables or fiber optics in a building which can be expensive and time-consuming. Thus there is a need for a system that can reliably transfer digital video streams across a common digital network backbone in a digital broadcasting environment that supports a large number of concurrent transfers.